I bought this kind of "upper beam headlights" for my bike: http://www.amazon.com/SecurityIng%C2%AE-3800Lm-Headlight-Headlamp-Bicycle/dp/B009QX8B2Q/ Maybe not the safest electronics* but dirt-cheap and bright as @$%&. Disregarding some safety issues* I ...

Step 5: Finished Part 1

Step 6: The Battery container

Waterproof, stores all the excess cable lengths and the "not-that-waterproof" connections in between, matches an average bottleholder you might already have installed on your bike, simply unscrew the ...

Against all my expectations this worked so fine as it had been made for that purpose... So see the next steps.

*Not waterproof as described and the low battery warning does not work, instead it suddenly switches off, leaving you in the dark. The included charger / power supply gets pretty hot if the battery is fully drained. You´ve been warned! But disregarding those 'minor' safety issues, it works fine ;)